A blend of a polycarbonate resin and a styrene-containing copolymer is a resin composition which has improved processability maintaining the good notched impact strength. The polycarbonate resin composition should further have good flame retardancy as well as high mechanical strength because the resin composition are applied to electric or electronic goods, automobile parts, office supplies, etc.
To provide the polycarbonate resin with good flame retardancy, a halogen-containing flame retardant and/or an antimony-containing compound were used. However, the disadvantages could be observed that the halogen-containing compound results in the corrosion of the mold itself by the hydrogen halide gases released during the molding process and is fatally harmful due to the toxic gases liberated in case of fire. Especially, since the polybromodiphenyl ether, mainly used for a halogen-containing flame retardant, can produce toxic gases such as dioxin or furan during combustion, flame retardants which are prepared without a halogen-containing compound have become a major concern in this field.
A general method is employ a phosphoric acid ester compound as a flame retardant to provide the polycarbonate resin with flame retardancy without using a halogen-containing compound. U.S. Pat. No. 4,692,488 discloses a thermoplastic resin composition comprising a halogen free aromatic polycarbonate resin, a halogen free SAN (styrene-acrylonitrile) copolymer, halogen free phosphorus compounds as flame retardants, a tetrafluoroethylene polymer and a small amount of ABS (acrylonitrile-butadiene-styrene) copolymer. If a halogen free phosphorus compound and a tetrafluoroethylene polymer are used together as in this U.S. patent, the dripping phenomenon can be prevented, but a juicing phenomenon occurs due to the migration of the flame retardant agent to the surface of the molded article during molding process.
U.S. Pat. No. 5,061,745 discloses a thermoplastic resin composition comprising an aromatic polycarbonate resin, an ABS graft copolymer, a copolymer, and a monophosphorous ester. As the flame retardant agent is monomeric, the juicing phenomenon is not prevented and the heat resistance is rapidly deteriorated.
It is known that an oligomeric phosphate is used as a flame retardant. Japanese Patent Publication No. 59-202,240 discloses a method of preparing an oligomeric phosphate and the use of the oligomeric phosphate in a polyamide or polycarbonate resin composition.
U.S. Pat. No. 5,204,394 discloses a resin composition comprising an aromatic polycarbonate resin, a styrene-containing copolymer or a graft copolymer, and oligomeric phosphate as flame retardants. Although the resin composition improves the juicing phenomenon and heat resistance, but is inferior to the resin composition using the monophosphorous esters as flame retardants in flame retardancy. Accordingly, to maintain a good flame retardancy, the resin composition should contain more flame retardants than in the resin composition containing monophosphorous ester as flame retardants. As a result, the resin composition shows poor mechanical properties due to the high content of a flame retardant. Furthermore, the resin composition contains still some monophosphorous ester which migrates to the surface of the molded article to cause juicing phenomenon in part.
U.S. Pat. No. 5,672,645 discloses a PC/ABS resin composition comprising an aromatic polycarbonate resin, a vinyl copolymer, a graft copolymer, a mixture of a monophosphate and an oligomeric phosphate, and a fluorinated polyolefin. In this case, the resin composition shows juicing phenomenon in which monophosphorous ester migrates to the surface of the molded article, and deterioration of heat resistance. And, as the flame retardability of an oligomeric phosphate ester is less than that of a momophosphorous ester in usual, the more oligomeric phosphate ester is contained in the flame retardant agent, the poorer flame retardancy of resin composition is.
Japanese Patent Laid-open No. 6-100,785 discloses a flame retardant resin composition which comprises a thermoplastic resin, a phosphate compound, and a silicon resin or a polyphosphazene to prevent from dripping of the resin during combustion. However, the Japanese patent application does not disclose that the flame retardancy of the resin composition had been improved.
EP 0 728 811 A2 discloses a thermoplastic resin composition comprising an aromatic polycarbonate, a graft copolymer, a copolymer and a phosphazene. The European patent teaches that no dripping occurs during combustion by using a phosphazene as a flame retardant even though an additional anti-dripping agent is not employed, and that the resin composition has excellent heat resistance and impact strength. However, the resin composition of the European patent has a poor processability due to the poor flowability by using the phosphazene, and causes black stripes and/or black lines on the surface of the molded article due to degradation of the resin or flame retardants when an excess stress is applied to the article during molding. In the European patent using phosphazene as a flame retardant, mechanical strength such as flexural strength and flexural modulus becomes deteriorated, and more amount of flame retardants should be used to maintain a certain degree of flame retardancy.
WO 00/09518 and WO 99/19383 disclose methods of preparing a cross-linked linear or cyclic phenoxy phosphazene and a thermoplastic resin composition using such phosphazene compounds. According to the patent applications, the cross-linked phenoxy phosphazene does not deteriorate the mechanical properties of the resin composition when applied, because the phosphazene has a high melting point and lower volatility. However, the resin composition of the patent applications has a poor processability due to the poor flowability by using the phosphazene, and causes black stripes and/or black lines on the surface of the molded article due to degradation of the resin or flame retardants when an excess stress is applied to the article during molding. In the patent applications using phosphazene as a flame retardant, mechanical strength such as flexural strength and flexural modulus becomes deteriorated.
The present inventors have developed a flame retardant thermoplastic resin composition that comprises a polycarbonate resin, a rubber modified vinyl-grafted copolymer, a vinyl copolymer, an oligomer type cyclic phosphazene compound and a phosphoric acid ester as a flame retardant and a fluorinated polyolefin resin, which has a good balance of physical properties such as impact strength, heat resistance, heat stability, processability and appearance.